Design and tests of the silicon sensors for the ZEUS micro vertex detector

To fully exploit the HERA-II upgrade, the ZEUS experiment has installed a Micro Vertex Detector (MVD) using n-type, single-sided, silicon μ-strip sensors with capacitive charge division. The sensors have a readout pitch of 120 μm, with five intermediate strips (20 μm strip pitch). The designs of the silicon sensors and of the test structures used to verify the technological parameters, are presented. Results on the electrical measurements are discussed. A total of 1123 sensors with three different geometries have been produced by Hamamatsu Photonics K.K. Irradiation tests with reactor neutrons and 60Co photons have been performed for a small sample of sensors. The results on neutron irradiation (with a fluence of 1×1013 1 MeV equivalent neutrons/cm2) are well described by empirical formulae for bulk damage. The 60Co photons (with doses up to 2.9 kGy) show the presence of generation currents in the SiO2–Si interface, a large shift of the flatband voltage and a decrease of the hole mobility.